mandos-client: Minor changes to check for more error conditions.

* mandos-client.c (add_remove_local_route): Do TEMP_FAILURE_RETRY
                                            around closing of
                                            /dev/null.  Check return
                                            value when opening helper
                                            directory and helper
                                            executable.
  (run_network_hooks): Cast return value of TEMP_FAILURE_RETRY to int.
                       Do TEMP_FAILURE_RETRY around closing of
                       /dev/null.